Fluidized bed with spray nozzle shielding

ABSTRACT

A shield, such as an upstanding cylindrical partition, is mounted adjacent to an air source, such as an air distribution plate/screen of a Wurster system processor, whereby the open upper end of the partition is generally horizontally registered with and disposed about the upper extremity of an associated spray nozzle. The open lower end of the inner cylindrical partition is generally sealed relative to the air distribution plate/screen and operative to receive air upwardly therethrough for subsequent passing through the inner partition about the spray nozzle. The upper end of the inner tubular partition shields the initial spray pattern discharged from the spray nozzle and prevents the premature entrance of particles moving into the spray nozzle area.

This is a Divisional of application Ser. No. 07/783,124, filed Oct. 28,1991 now U.S. Pat. No. 5,236,503, dated Aug. 17, 1993.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a fluidized bed system having a spray nozzletherewithin. One such system is a Wurster system type of fluidized bedprocessor including an outer cylindrical partition disposed within theproduct container and a spray nozzle within the partition. The spraynozzle itself is shielded, preferably by being surrounded by acylindrical partition extending from the orifice plate or screen at thebottom end of the product container to shield the spray nozzle tip.

2. Description of Related Art

Various different forms of spray coaters heretofore have been providedincluding the Wurster system type. These previously known devices mayexperience turbulent air flow immediately above the spray nozzle.Particles to be coated within the system may enter the liquid dischargespray pattern before the spray pattern has fully developed. This resultsin uncontrolled droplet formation upon those particles that enter thespray pattern too soon and effects the effectiveness of the systemresulting in excessive agglomeration and relatively extensive processingtime.

U.S. Pat. No. 3,110,626 to G. L. Larson et al. discloses an apparatuswhereby coating discrete solids suspended in a moving air stream iscarried out within the interior region of a velocity concentrationcontrol element mounted in the base region of a funnel-shaped coatingchamber. However, such apparatus does not include any means forshielding the base of the spray pattern with an upwardly flowing columnof air in order that the spray pattern may substantially develop beforeentrance thereinto of discrete solids to be coated.

U.S. Pat. No. 4,335,676 to Christian Debayeux et al. discloses a spoutedbed granulating and/or coating apparatus wherein flow directingstructure is provided to direct the gaseous flow stream in the upwarddirection for preventing contact and agglomeration of particles in thevicinity of the walls of the device. This patent fails to disclosestructure by which the lower portion of the spray pattern is protectedby an upwardly flowing column of air in order that the spray pattern maymore fully develop before the entrance thereinto of particles to becoated.

U.S. Pat. No. 4,701,353 to Stanislaus M. P. Mutsers et al. discloses anapparatus whereby the liquid spray material is discharged out of acentral channel as a vertically closed, conical film with a thrustexceeding the thrust of the gas stream for the purpose of causing theconical film to be nebulized to very fine droplets with the aid of thesurrounding gas stream. The resultant spray pattern is not protectedabout its initial base end by an upwardly moving column of air disposedthereabout.

U.S. Pat. No. 4,960,224 to Gustav A. Magg et al. discloses an atomizingnozzle constructed in a manner to eliminate the need to provide ametering pump or flow meter for each atomizing nozzle of an associatedfluidized coating bed with the control of the flow through eachatomizing nozzle being accomplished by varying the internal bore size ofthe flow control tubes. However, this patent fails to disclose structurefor shielding the resultant spray pattern from immediate entrancethereinto of particles to be coated before the spray pattern isreasonably developed.

U.S. Pat. No. 4,858,552 to Werner Glatt et al. discloses an apparatuswhereby a fluidized current carries particles, while still plastic,upwardly through a channeling device for agglomerated material disposedat a distance above the perforated base causing the particles to impingeon the underside of a rotatable means providing for shaping theagglomerated material. The Glatt et al. apparatus does not disclosestructure by which the particles to be coated are shielded against entryinto the initially forming spray pattern.

U.S. Pat. No. 3,196,827 to D. E. Wurster et al. discloses a tubularpartition defining an upbed therein into which an air and spraydischarge pattern is directed and wherein a downbed of particles in nearweightless suspension is disposed outwardly of the tubular partition,the spray nozzle being disposed below the bottom of the partition andabove the associated air distribution plate or screen. With this device,particles being coated are also free to immediately enter the lowerbeginning portion of the spray pattern.

SUMMARY OF THE INVENTION

It is understood that the dynamics of the area around the nozzle andspray cloud (often described as the coating zone) determine the overallcoating speed as well as the amount of agglomeration during a fluidizedbed coating process. The type of nozzles commonly used are pneumaticallyatomized, i.e. using a high-speed jet of air in order to break a liquidjet into small droplets and to distribute the small droplets in acone-shaped cloud or spray pattern.

It has been observed in the course of several laboratory Wurster coatingtrials that material, including substrate, from the fluidization orprocessing air stream has been drawn into the spray nozzle liquid/airjet before the spray pattern has been fully developed. In some cases,when the material being coated has abrasive properties, it was foundthat the material was moving with sufficient force to cause erosion ofthe nozzle tip.

Accordingly, the present invention introduces a shielding or barriermeans around the lower portion of the nozzle, within the upbed, forshielding the nozzle and allowing up flow of air within the shieldingmeans around the nozzle. This ensures that particles, disposed in theproduct container outwardly of the barrier, are prevented from enteringthe spray pattern before the spray pattern is sufficiently developed.This allows the droplet density to decrease before contact therewith bythe particles of the fluidized bed and, accordingly, the particlesurface will be more evenly wetted preventing excessing particleagglomeration. The liquid contact with the particles can be moreprecisely controlled and higher spray rates can be achieved with lessagglomeration.

A principal object of this invention is to shield the spray dischargingnozzle of a Wurster type fluidized bed processor. The shield preventsthe entry of particles into the spray pattern before the spray patternhas had an opportunity to develop.

Another object of this invention is to provide an apparatus by whichparticles to be coated are prevented from entering the spray patternuntil such time as the droplet density of the spray pattern has beensubstantially reduced.

Yet another object of this invention is to provide a columnar shield ofupwardly moving air about the lower portion of the spray pattern formedby a spray nozzle of a Wurster system processor and wherein the shieldmay be adjusted vertically according to the spray pattern beingdischarged and the air flow velocity of the processing air.

A further object of this invention is to provide a shield such as thatset forth in the immediately preceding object and whereby the verticalpositioning of the shield may be utilized to alter the associated spraypattern.

Yet another object of this invention is to provide a coating zone withinthe upbed of a Wurster system coater whereby the coating zone isprotected by a surrounding column of upwardly moving air in order toallow the coating zone to more fully develop and the liquid dropletdensity thereof to be substantially reduced prior to entry of particlesinto the coating zone.

These, together with other objects and advantages which will becomesubsequently apparent, reside in the details of construction andoperation as more fully hereinafter described and claimed, referencebeing made to the accompanying drawings forming a part hereof, whereinlike numerals refer to like parts throughout.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary schematic vertical sectional view of aWurster-type bottom spray coater illustrating the mounting of an innertubular partition about the spray nozzle and projecting at leastslightly above the upper extremity of the spray nozzle; and

FIG. 2 is an enlarged fragmentary schematic vertical sectional viewillustrating the manner in which the interior partition may be adjustedvertically relative to the air distribution plate.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now more specifically to the drawings the numeral 10 generallydesignates a typical Wurster system type of coater, modified inaccordance with the present invention. The coater includes a productcontainer section 12, an expansion chamber 14 into which the upper endof the product container section 12 opens, and a lower plenum 16disposed beneath the product container, separated therefrom through theutilization of an air distribution plate or screen 18. The upper end ofthe expansion chamber 14 may open into a filter housing (not shown)disposed thereabove including the necessary air filter structure and airoutlet. A typical Wurster coater is described in Encyclopedia ofPharmaceutical Technology, Vol. 1. pp. 192-195 (1988).

The air distribution plate or screen 18 defines a plurality of airpassage openings 20 through which air or gas from the lower plenum 16may pass into the product container section 12. As is conventional inWurster-type fluidized bed systems, the holes in the plate 18 in thearea outside the cylindrical partition 22 are smaller in diameter thanthe holes beneath the partition 22. This results in higher air volumesand velocities in the central area than in the downbed area. Althoughonly a single plate 18 is depicted, it should be understood that theplate 18 is typically formed of two plates, an outer annular plate andan inner replaceable plate whereby the air flow ratios between the upbedand the downbed may be changed by substituting the inner plate with aplate having different air opening configurations. Further, the innerplate may itself be formed of two stacked plates having identical airopening configurations but which are rotatable with respect to eachother to change the effective open area.

The product container 12 has a cylindrical partition 22 supportedtherein in any convenient manner having open upper and lower ends, 24and 26, the lower end 26 being spaced above the air distribution plateor screen 18. The partition 22 divides the interior of the productcontainer section 12 into an outer annular downbed area 28 and interiorupbed 30. A spray nozzle assembly, referred to generally by thereference number 32, is mounted at or through the air distribution plate18 and preferably projects upwardly into the interior of the cylindricalpartition 22 and the upbed 30 defined therein. The spray nozzle 32receives a supply of air under pressure through an air supply line 34and coating liquid under pressure through a liquid supply line 36, as isknown in the art.

This invention incorporates the provision of a shield, such as an innercylindrical partition 40 disposed about the upwardly projecting spraynozzle assembly 32. The inner cylindrical partition 40 has its loweropen end snugly and telescopingly received within a tubular collar 42secured to the air distribution plate or screen 18 about the opening 44through which the spray nozzle assembly 32 is secured. The tubularcollar 42 is secured to the air distribution plate or screen 18 throughthe utilization of suitable fasteners 46. The collar 42 includescircumferentially spaced axial slots 48 in which mounting studs 50projecting radially outwardly of the inner cylindrical partition 40 areslidingly received. The mounting studs 50 have threaded bolts 52threadedly engaged therewith whereby vertically adjustment of the innercylindrical partition 42 relative to the tubular collar 42 may beenabled.

The spray nozzle assembly 32 discharges a spray pattern 56 of air andcoating liquid. Some of the air introduced into the lower plenum 16passes upwardly through the openings 20 formed through the airdistribution plate or screen 18 below the inner cylindrical partition40. The partition 40, as well as the tubular column of upwardly risingair about the spray nozzle assembly 32, shields the lower, beginningportion, of the spray pattern 56. The particles 60, passing upwardlythrough the upbed 30, are not drawn into this spray pattern. The annularcolumn of air thereby allows spray pattern 56 to substantially develop,and the liquid droplet density of the spray pattern is substantiallyreduced before particles 60 enter into the spray pattern 56. Byadjusting the height of the inner cylindrical partition 40 relative tothe tubular collar 42, the height of the upper end of the innercylindrical partition 40 relative to the upper extremity of the spraynozzle assembly 32 may be adjusted.

Although only a single outer partition 22, inner partition 40, and spraynozzle 32 are depicted, multiple outer partitions may be employed, eachhaving one or more spray nozzles and inner partitions associatedtherewith.

Various modifications to the above-described preferred embodiment may beutilized. For example, a spray nozzle may be utilized in systems otherthan Wurster coater type systems where shielding is desirable. Suchsystems may not require the spray nozzle to be upwardly disposed; thespray nozzle may be angled with respect to the major axis of thecontainer. The shielded spray nozzle may also be utilized without aWurster-type cylindrical partition 22. The shielded spray nozzle mayalso be within an expansion chamber instead of the product container.Further, although an air distribution plate or screen is depicted, gasflow may originate into the processor through other structuralarrangements. In addition, although the shielding of the spray nozzle ispreferably provided by an inner cylindrical partition 40, othershielding arrangements may be utilized. For example, shielding of thespray nozzle may be accomplished by formation of an air wall or streamthat surrounds the nozzle and prevents particles from prematurelyentering into the spray pattern. Alternatively, a deflector or shieldmay be formed integral with the spray nozzle itself to prevent particlesfrom interfering with the developing spray pattern.

Utilizing the concepts of the invention hereinabove described, thefollowing represents test procedures followed and results obtained madein accordance with the invention:

Materials

Sugar/cornstarch beads (Nu-pariels, Crompton & Knowles, IngredientTechnology Division, Pennsauken, N.J.) in the size range of 20-25 meshwere used as a model coating substrate for most of the trials. Thesebeads are deemed to be a good model since many active materials areconverted into pellet form for coating, or are layered onto these typeof beads. Theophylline (<325 mesh, Knoll Whippany, N.J.) and potassiumchloride (20-60 mesh, Mallinkrodt, St. Louis, Mo.) served as models forpowder substrates.

A typical aqueous HPMC based coating solution (Opadry, Colorcon WestPoint, Pa. 10% coating by weight in tap water) was used throughout thetrials. A green colored formula (YSl-3303N) was used for most of theexperiments but was shifted to a Maroon color (Y-1-3910) to check theeffects of viscosity and composition differences on the coatingperformance.

Equipment

All of the coating trials were carried out in a GPCG60/100 fluid bedgranulator/coater (Glatt Air Techniques, Ramsey, N.J.) using either the32" or 18" Wurster process inserts. The 18" Wurster trials utilized thestandard 9" diameter partition insert, which is 22" long. The 32"Wurster trials were performed with a single partition insert 12" indiameter, 27.5" long.

The partition height was adjusted so that a thick stream of substrateflowed through the coating zone. Due to the different airflow patternsin the two sizes of Wurster inserts used the partition needed to be setat a different height for each insert. A distance of 0.75 inches fromthe bottom plate was sufficient for the 18" Wurster. For the 32" Wursterthe partition was raised to 1.25 inches above the bottom plate tomaintain a high density particle stream.

A bottom screen, stainless steel dutch weave type, 100 mesh rating wasin place over the air distribution plate during all the trials. Exhaustfilters, "PACF" type, rated 3-10 microns were used to retain any processfines. The filter shake cycle was set to shake for 3 s every 30 secondsin the GPCG mode. This meant that each filter shook 3 s after every 63 sof process time.

Two types of nozzles were used each with a different characteristicspray pattern development. Model #940/7-1-S25 (manufactured by GustavSchlick Co. Coburg, Germany referred to as S-25) is supplied standardfor these sizes of Wurster inserts. Model #0/4-7-1S48 (referred to asS-48) has a higher air consumption rate and fully atomizes the liquidstream distributing the droplets in a cone shaped pattern (developsspray pattern) in a shorter distance. Liquid was delivered to thenozzles with a peristaltic pump. The atomization air pressure wasadjusted so that the droplet size distribution would remain the same nomatter which nozzle was in use. Nozzle ports with a 1.2 mm opening wereused with the air cap adjusted flush with the nozzle port end.

In those trials where the fluid flow in the coating zone was altered thefluidized particles were delayed from contacting the atomized stream bya 3" diameter cylinder partition mounted on the bottom plate andextending up 0.5" above the nozzle port height. The cylinder partitionwas open to the fluidizing air at the bottom.

Test Procedure

The air volume settings were determined by a pre-trial fluidizationtest. Air volume was then held constant for each unit throughout thetrials without further optimization. For the 18" Wurster the air volumewas 750 cfm (±100 cfm) and 1700 cfm (±100 cfm) was used on the 32"Wurster. For experiments on both inserts the product temperature washeld between 38° and 42° C. with an inlet air dewpoint of 8° C. (±1°C.).

Each run would be allowed to come to the desired test conditions, thenthe spray was started at a slow rate. If after 15 minutes of spray asample taken had less than 1% retained on the sieve specified for thetest then the performance was deemed acceptable and the spray rate wouldbe raised. The amount of the spray increase was determined by estimatingthe point at which 1% of the sample would be retained on the test sieve.

With the sugar beads a 20 mesh sieve was the test sieve as mentionedabove. After each trial the batch was put through a 20 mesh sieve inorder to remove any agglomerates. The beads passing through 20 mesh wereused in the subsequent trial. For the 32" trials with sugar beads an 18mesh sieve was used in the test in order to account for the bead growthdue to repeated coating. On the 18" Wurster a starting charge of 45 kgwas used compared to 200 kg on the 32" Wurster.

The experiment with theophylline as a substrate used 26 kg of rawmaterial in the 18" Wurster. The test sieve for Theophylline was 100mesh. With potassium chloride 50 kg were charged to the productcontainer. At the end of the potassium chloride run the processparameters were varied in order to find the maximum possible spray ratewithout having more than 1% on 18 mesh. Air volume was increased fromthe protocol value of 750 cfm to the highest value possible, 1200 cfm.Atomization air pressure was also increased from 2.5 bar to 4.0 bar.

Experimental Program

A matrix of experiments was set up to test the coating performance withnozzle type, barrier installation, and machine size as variables. On the18" Wurster four experiments were run with the sugar beads and the greencoating: nozzle S-25 with and without the barrier in place, and nozzleS-48 with and without the barrier in place. The set of experiments usingthe S-25 nozzle was then repeated in order to confirm the results.Powdered substrate, theophylline and potassium chloride were used withthe S-48 nozzle and the barrier in place in the last two experiments onthe 18" Wurster.

In the 32" Wurster two experiments using the S-48 nozzle and sprayingonto sugar beads were performed--one with the barrier in place and onewithout. In these trials the coating solution was switched to maroon tosee if the improvements in coating efficiency were still evident with amore viscous solution.

Results and Discussion

The spray rate at 1% agglomeration was estimated from the data reportedduring the run on the batch sheets. Data for the matrix of 18" Wursterexperiments is shown in Table 1. The data for the S-48 nozzleexperiments has been averaged between the two runs.

                  TABLE 1                                                         ______________________________________                                        18" Wurster                                                                   Spray Rate @ 1% Agglomeration                                                 (ml/min)                                                                      nozzle        barrier no barrier                                              ______________________________________                                        S-25          290      90                                                     S-48          410     270                                                     ______________________________________                                    

Examination of the table reveals a dramatic improvement in spray rate atan equal level of agglomeration for use of a barrier as compared withthe standard case without use of a barrier. It is also of interest thatthe improved efficiency by changing to a nozzle that develops the spraypattern more quickly is nearly the same as placing a barrier around thestandard nozzle. This observation is in agreement with the hypothesisthat the agglomeration and limitation of spray rate is caused by thepremature contact of substrate with the developing spray pattern.

With a similar extraction of the data the 32" Wurster showed 1%agglomeration at 550 g/min spray without the barrier and 780 g/min withthe barrier in place. Only the S48 nozzle was tested this time with themaroon coating. The efficiency increase is in similar proportions to theexperiment on the 18" Wurster even though the scale of the machine andthe coating characteristics were changed.

Both the theophylline and potassium chloride coated with the S-48 nozzleand the barrier in place had similar behavior during the run to aconventional machine setup. The spray rate for the potassium chloridesubstrate was 1050 g/min at the point of first agglomeration.Agglomeration began to occur slowly at 1250 g/m. However, at this highspray rate, the machine's drying capacity was exceeded and the desiredproduct temperature could not be maintained. As a result, excessivesurface moisture probably allowed the formulation of liquid bridgesbetween particles ultimately leading to agglomeration. Although theacceptable spray rate for theophylline was less dramatic, the 230 g/minsprayed was higher than expected and yet did not exceed an acceptablelevel as in the sugar bead experiments.

The surprising result of these experiments was the ability to coat thesmall particle size material without granulating the substrate.Comparative SEM's at the same magnification show the small fraction ofthe particle size distribution being incorporated into the coating onthe large fraction of the distribution but then the material ceases toagglomerate and at 10% coating the particle size is very similar to the4% sample. The average size of the end coated particles beingsignificantly less than 100 microns, the current accepted lower limitfor discrete fine particle coating. It was postulated from thisobservation that the minimum particle size for discrete coating might beeffected by the substrate/spray pattern contact and could besignificantly reduced by limiting that contact while the spray patternis still developing.

CONCLUSIONS

Throughout the experiments described here it is clear that limiting thecontact of substrate with a developing spray pattern so that theparticle surface is only exposed to spray in low droplet density areasof the spray pattern reduces the over wetting of substrate surfacethereby reducing the rate of product agglomeration. Introduction of abarrier to prevent solids from entering the spray pattern and changingthe nozzle design so that the pattern develops in a smaller space wereshown to have a roughly equivalent effect. Substantial efficiencyimprovements were still evident when the barrier was used with themodified nozzle. This technique of improved solid liquid contacting wasobserved on two different sizes of equipment with two different coatingformulations on three different substrates.

In addition to the expected result of improved coating efficiency it wasalso observed for the theophylline, the sample with smallest particlesize, that the particle size of the coated material could be kept belownormally accepted minimum sizes.

What is claimed is:
 1. A method for coating product in a fluidized bedhaving a product container section opening upwardly into an expansionchamber and downwardly into a lower plenum chamber through a generallyhorizontally disposed air distribution plate/screen having openingsformed therethrough for upward air flow from said lower plenum chamberinto said product container section, said product container sectionincluding a substantially cylindrical partition spaced above said airdistribution plate/screen for dividing said product container sectioninto an inner upbed area and an outer downbed area, and an upwardlydischarging spray nozzle mounted substantially centrally within saidcylindrical partition, said method including the steps of positioning acylindrical inner partition adjacent said distribution plate/screen andextending upwardly therefrom, surrounding said nozzle, and projectingupwardly to a level at least equal in height to said nozzle, and passingair upwardly through said air distribution plate/screen and through saidcylindrical inner partition about said nozzle to shield the initialspray pattern developed by said nozzle against the entrance of particlesmoving upwardly through said upbed.
 2. The method of reducing theprocessing time of a granulator/coater of the fluidized bed typeincluding a product container section opening upwardly into an upperexpansion chamber and downwardly into a lower plenum chamber through agenerally horizontal air distribution plate/screen having openingsformed therethrough for upward air flow from said lower plenum chamberinto said product container section and wherein said product containersection contains an upright cylindrical partition supported centrallytherein spaced above said air distribution plate/screen and dividingsaid product container section into an inner upbed and an outer downbed,and an upwardly discharging spray nozzle mounted centrally with respectto said upbed in a lower portion thereof, said method including thesteps of forming a radially confined and shielded column of air to flowupwardly about said nozzle from said air distribution plate/screen andto be freely discharged into said upbed at an elevation generallyvertically registered with the upper extremity of said nozzle.
 3. Themethod of claim 1 wherein the shielding step comprises providing abarrier by positioning a partition about the spray nozzle, saidpartition having opposite open ends, one open end of said partitionpositioned away from the spray nozzle tip toward the direction of thedischarging spray, the other open end of said partition positioned tosurround a portion of the spray nozzle.
 4. A method for shielding aninitial spray pattern developed by a discharging spray nozzle situatedwithin a fluidized bed processor, said fluidized bed processor includinga container for containing particles to be processed, a fluidizing gassource, and a discharging spray nozzle having a spray nozzle tip withinthe container, the method comprising the steps of forming a fluidizedbed within the container by passing fluidizing gas through the particlesto be processed, discharging a spray from the discharging spray nozzleinto the fluidized bed, and shielding the initial spray patterndeveloped by the discharging spray nozzle by providing a barriersurrounding the spray nozzle tip and oriented substantially parallel tothe spray nozzle for preventing the particles to be processed fromentering the initial spray pattern.
 5. The method of claim 1, whereinthe shielding step comprises providing a barrier formed from a column ofgas about the discharging spray nozzle tip and directed substantiallyparallel with the spray nozzle, said column of gas extending beyond thespray nozzle tip in the direction of the discharging spray.
 6. Themethod of claim 5, wherein the column of gas is formed from thefluidizing gas.
 7. The method of claim 5, wherein the column of gas issubstantially radially confined.
 8. The method of claim 7, wherein theforming of the substantially radially confined column of gas comprisespositioning a cylindrical partition having opposite open ends around thespray nozzle, passing gas through one end of the cylindrical partitionand outward through the other end of the cylindrical partition, saidother end positioned adjacent the spray nozzle tip, whereby the columnof gas prevents the particles being processed from entering the initialspray pattern.
 9. The method of claim 8, wherein the column of gas isformed from the fluidizing gas.